Sulfur occurs in many industrial processes, and sulfur, or sulfur containing compounds, because sulfur is a catalyst poison and is environmentally unacceptable must invariably be removed from process streams, e.g., flue gas, waste gas or recycle gas streams. This has been accomplished, e.g., by contacting the sulfur-containing process stream with a sorbent comprising a particulate oxide, hydrated oxide, or hydroxide of alumina, zinc, nickel, cobalt or the like, alone or in admixture with each other or with additional materials, e.g., alkali or alkaline earth metal oxides or the like. The quality of sorbents for sulfur removal varies considerably, and in many applications it is essential to scrub essentially all of the sulfur from the process streams.
Sulfur finds its way into a process principally via the feed, and essentially all petroleum feeds contain sulfur. Catalytic reforming, or hydroforming, is exemplary of a well-known and important process employed in the petroleum refining industry for improving the octane quality of naphthas and straight run gasolines, and it is illustrative of a process where the presence of sulfur can have a detrimental effect. Sulfur can gradually accumulate upon and poison the catalyst. In a typical process, a series of reactors are provided with fixed beds of catalyst which receive upflow or downflow feed, and each reactor is provided with a heater, or interstage heater, because the reactions which take place are endothermic. A naphtha feed, with hydrogen, or recycle gas, is concurrently passed sequentially through a reheat furnace and then to the next reactor of the series. The vapor effluent from the last reactor of the series is a gas rich in hydrogen, which usually contains small amounts of normally gaseous hydrocarbons, and it is separated from the C.sub.5.sup.+ liquid product and recycled to the process to minimize coke production; coke invariably forming and depositing on the catalyst during the reaction.
Reforming catalysts are recognized as dual functional, the catalyst composite including a metal, or metals, or a compound or compounds thereof, providing a hydrogenation-dehydrogenation function and an acidic component providing an isomerization function. Conventional reforming catalysts have thus long employed platinum composited with an inorganic oxide base, particularly alumina, and in recent years promoters such as iridium, rhenium, germanium, tin, etc., have been added, particularly to platinum, to enhance one or more of certain of the characteristics which a good reforming catalyst must possess--viz., activity, selectivity, activity maintenance and yield stability. Halogen, e.g., chlorine, is generally added to enhance the acid function required of the catalyst.
The principal reactions produced in reforming are: (1) dehydrogenation of naphthenes to produce the corresponding aromatic hydrocarbons, e.g., methylcyclohexane is dehydrogenated to form toluene, (2) isomerization of n-paraffins to form branched-chain paraffins and isomerization of ring compounds, e.g., the isomerization of ethylcyclopentane to form methylcyclohexane, and dehydrogenation of the latter to form toluene, (3) dehydrocyclization of paraffins to form aromatics, e.g., the dehydrocyclization of n-heptane to form toluene, and (4) hydrocracking high molecular weight feed constituents to form lower molecular weight, or lower boiling, constituents, e.g., the cracking of n-decane to produce C.sub.3 and C.sub.7 hydrocarbons. The net effect of these reactions is to increase the concentration of aromatics and isomers, with consequent octane improvement of naphthas boiling within the gasoline range.
In use of the more recently developed polymetallic platinum catalysts wherein an additional metal, or metals, component is added as a promoter to the platinum, it has become essential to reduce the feed sulfur to only a few parts, per million parts by weight of feed (ppm), because of the sulfur sensitiveness of these catalysts. For example, in the use of platinum-iridium and platinum-rhenium catalysts it is generally necessary to reduce the sulfur concentration of the feed well below about 10 ppm, and preferably well below about 2 ppm, to avoid excessive loss of catalyst activity and C.sub.5.sup.+ liquid yield. Generally, adequate sulfur can be removed from such feeds by hydrofining; hydrofining being followed by passage of the hydrofined product through guard chambers packed with metal oxides, e.g., nickel or cobalt oxide, to remove residual amounts of sulfur from the hydrofined product.
The sulfur must also be scrubbled from the hydrogen recycle stream because this too is a source of catalyst sulfur contamination. The vapor effluent from the last reactor of the series is thus a gas rich in hydrogen, which can contain hydrogen chloride, chlorine, hydrogen sulfide, moisture and small amounts of normally gaseous hydrocarbons. It is essential to separate hydrogen from the C.sub.5.sup.+ liquid product and recycle it to the process; and it is essential to remove the sulfur from the recycle hydrogen gas stream.
An admirably useful sorbent for the removal of sulfur from hydrogen recycle gas streams is disclosed in U.S. Pat. No. 4,263,020 which issued Apr. 21, 1981, to Paul E. Eberly, Jr. The sorbent disclosed therein is constituted of a particulate mass of metal alumina spinel, MAl.sub.2 O.sub.4, wherein M is chromium, iron, cobalt, nickel, copper, cadmium, or mercury, particularly zinc alumina spinel, ZnAl.sub.2 O.sub.4. In a preferred operation, a particulate mass of metal alumina spinel, notably the zinc alumina spinel, is charged, or packed into a guard chamber, or series of guard chambers. Suitably the series of metal alumina spinel guard chambers are employed in parallel, this permitting active use of one guard chamber, or set of serially aligned guard chambers for contact, and purification of the recycle hydrogen stream while the other guard chamber, or set of serially aligned guard chambers, is cut out of series for regeneration. In the treatment of a hydrogen recycle gas stream, as employed in reforming, it is found that the hydrogen sulfide can be readily adsorbed from the stream despite the high moisture content of the gas which typically contains up to about 50 vppm (parts per million, based on volume) hydrogen chloride and water, and up to about 25 vppm hydrogen sulfide. Moreover the metal alumina spinel, or zinc alumina spinel, is readily regenerated by simply purging, or sweeping the sulfur compound therefrom with a hot, non-reactive, or inert gas, e.g., hydrogen, or hydrogen-containing gas, after the zinc spinel has become sufficiently saturated with the sulfur compound. The zinc alumina spinel, is simply contacted, purged, or swept with, e.g., the hydrogen gas stream at elevated temperature ranging from about 300.degree. F. to about 1200.degree. F., or preferably from about 500.degree. F. to about 1000.degree. F., to remove the hydrogen sulfide, and other sulfur compounds, and thereby regenerate the zinc alumina spinel.
Albeit, this process has performed admirably, i.e., is regenerable and has provided surface breakthrough capacities many times greater than conventional sorbents, it is nonetheless desirable to further improve the sulfur capacity of the metal alumina spinel sorbents.